Page 646 - Air and Gas Drilling Manual

12-10 Air and Gas Drilling Manual
These
tools
available
electromagnetic
either
are
MWD
with
magnetometer/accelerometer-based survey subsystems or with gyroscope based
survey subsystems. The three-axis magnetometer and accelerometer survey
subsystem must utilize nonmagnetic housings and drill collars in much the same
manner as the downhole magnetic survey instruments discussed above. The
alternative gyroscope based survey subsystem utilizes two, two-axis gyroscopes, and
three-axis accelerometers. This gyroscope based subsystem does not require
nonmagnetic housings or drill collars.

Figure 12-3: Schematic of an electromagnetic MWD transmission of data to a surface
antenna (courtesy Geoservices Inc.).
Air and Gas Drilling Applications
The electromagnetic MWD system has incorporated several other important
downhole logging features that are of particular use in underbalanced drilling
operations. These are bottomhole annulus pressure, gamma ray, and resistivity
measurements. The mud pulse transmission based MWD systems have all but
totally replaced the steering tools. Since steering tools still have limited use in
larger diameter borehole air and gas directional drilling operations. Therefore,
steering tools have not generally been upgraded to carry out auxiliary logging
operations (e.g., bottomhole pressure, etc).
Both the steering tool system and the electromagnetic MWD system have
durability problems related to the drill string high vibration environments that
characterizes compressed air (or other gas) drilling operations. This is basically a